For those who haven't come to Mykonos, I suggest putting this near the top of your list. I've been going for 18 years and I always have a good time. I also always hate to leave, and that's the mark of a great destination.
Mykonos offers gorgeous beaches (of all sorts party spots, secluded coves, singles havens, family enclaves, gay/lesbian spots, etc.), a super charming town that's wonderful for evening promenades, excellent restaurants (Greek, Italian, Continental, Japanese, and more) with a good dose of fresh seafood, boutique shopping (consider well-crafted gold jewelry), and nightlife for everyone from the person wanting a little night cap to heavy club partiers.
This Greek island has a cool vibe that I've rarely seen elsewhere maybe because of all the chic Italians and Greeks who wear smart clothes to match their attractive looks. There's a reason a popular bar here is called Jackie O.
The typical day starts with a late breakfast at 11am. One reason for this is that the sun is hottest from 12 to 2, so most people show up at a beach around 1 or 2, not earlier, and then spend a few hours at one of Mykonos' many beautiful stretches of sand, taking lunch at one of the many beach club restaurants. Some of my favorite beaches include Psarou (chic, trendy may need to reserve a lounge chair), Elia (many singles and couples), Panormous (quiet, secluded), and Super Paradise (lots of the young and fashionable; try Coco Bar for a lovely lunch and awesome views). Also search out Kiki's for lunch one day (above Agios Sostis and near Panormous).
I like to swim for 40 minutes after lunch. Afterall, you can't just drink and eat all day, and lay on a beach!
To get to the beaches, it's best to rent a car, although its cheaper to take a bus to Platis Gialos then there are boats to several beaches. In my younger days, I rented a scooter however, these days a car seems safer. There are hundreds of scooters zipping around the island.
Dinner in Mykonos is more common at 10pm. Sure, Americans like to eat earlier but most people eat between 9:30 and 10pm, or even 11pm. My favorite spots include Gola (amazing food), Nammos (on Psarou beach) with its beautiful setting and super-fresh fish, and Sale Pepe for top-notch Italian. These three are not to miss. A simple, small Greek taverna is Magiori for excellent meat dishes.
Finally, if you can stay awake, head out to one of the various bars for a drink. The town is definitely buzzing at midnight (shops are even open)! Try Pieros, the Mykonos Bar, or one of the bars at the waterfront (like Babylon).
As for hotels, one has to decide whether to be in town (which I recommend) or out of town (perhaps at one of the beaches). My favorite and a smart splurge in town is the Semeli hotel. It's chic but still relaxed, and the staff is very friendly, plus the setting is charming. There's the Belvedere next door, but I don't feel it's worth the premium price versus the Semeli.
An ultra-luxe resort is Cavo Tagoo. It's beautiful and larger than the Semeli (and has more spacious rooms), but it's much pricier and the location is not as convenient. At least go here one evening for sunset drinks.
I should add that it's possible to visit ancient ruins on the nearby island of Delos. While not everyone spends each day at a beach, I have yet to make it there. I'm drawn to the magic of Mykonos and never want to leave the island!
Tip: Bring your most fashionable sunglasses to Mykonos. After you arrive, be prepared to buy another pair boasting whatever happens to be the latest look on the island!